Method of making shaped ceramic composites

ABSTRACT

The present invention provides a method for producing a self-supporting ceramic composite by the oxidation of a parent metal to form a polycrystalline ceramic material consisting essentially of the oxidation reaction product of the parent metal with an oxidant, including a vapor-phase oxidant, and, optionally, one or more metallic constituents. A permeable filler material, such as a preform, with at least one surface bearing a permeable stratum, is contacted with a body of molten parent metal heated to a temperature above its melting point but below the melting point of the oxidation reaction product. At least a portion of the oxidation reaction product is maintained in contact with and between the molten metal and oxidant to transport the molten metal through the oxidation reaction product toward the permeable stratum and into contact with the oxidant so that the oxidation reaction product continues to form at the interface between the oxidant and previously formed oxidation reaction product that has infiltrated the filler material. The reaction is continued to infiltrate at least a portion of the stratum with the oxidation reaction product and to produce an intermediate ceramic body having an adjacent ceramic composite overlaid with a ceramic stratum. The ceramic stratum is removed from the underlying ceramic composite to produce a self-supporting ceramic composite having the surface established by the permeable stratum.

This is a continuation of co-pending application Ser. No. 207,924 filedon June 13, 1988, now U.S. Pat. No. 4,824,622 which in turn is acontinuation of U.S. application Ser. No. 945,404 filed on Dec. 22,1986, now abandoned.

FIELD OF THE INVENTION

This invention broadly relates to methods for producting self-supportingceramic composites. More particularly, this invention relates to methodsof making self-supporting ceramic composites by the directed growth ofan oxidation reaction product of a parent metal into a permeable mass offiller material and into an adjacent permeable stratum outwardlydisposed to the mass of filler, such that the resulting compositestratum has a mechanical integrity weaker than the resulting compositemass of filler and is separable therefrom, thereby establishing aboundary to the infiltrated mass.

DESCRIPTION OF COMMONLY OWNED PATENT APPLICATIONS AND BACKGROUND

The subject matter of this application is related to copending andCommonly Owned U.S. Patent Applications, which include the following:Ser. No. 818,943, filed Jan. 15, 1986, which issued on Dec. 15, 1987, asU.S. Pat. No. 4,713,360 and its ancestor applications (now abandoned),all in the name of Marc S. Newkirk et al. and entitled "Novel CeramicMaterials and Methods for Making the Same". These applications disclosethe method of producing self-supporting ceramic bodies grown as theoxidation reaction product from a parent metal precursor. Molten parentmetal is reacted with a vapor-phase oxidant to form an oxidationreaction product, and the metal migrates through the oxidation reactionproduct toward the oxidant thereby continuously developing apolycrystalline ceramic body of the oxidation reaction product. Theceramic body can be produced having metallic components and/or porosity,which may or may not be interconnected. The process may be enhanced bythe use of an alloyed dopant, such as in the case of an aluminum parentmetal oxidized in air. This method was improved by the use of externaldopants applied to the surface of the precursor metal as disclosed inCommonly Owned and Copending U.S. patent application Ser. No. 220,935,filed June 23, 1988, which issued on Aug. 1, 1989, as U.S. Pat. No.4,853,352, which is a continuation of Application Ser. No. 822,999,filed Jan. 27, 1986, and its ancestor applications (now abandoned), allin the name of Marc S. Newkirk et al. and entitled "Methods of MakingSelf-Supporting Ceramic Materials".

The subject matter of this application is also related to that ofCommonly Owned and Copending U.S. patent application Ser. Nos. 819,397,which issued on July 25, 1989, in U.S. Pat. No. 4,851,375 filed Jan. 17,1986, which is a continuation-in-part of Ser. No. 697,876, filed Feb. 4,1985 (now abandoned), both in the name of Marc S. Newkirk et al. andentitled "Composite Ceramic Articles and Methods of Making Same". Theseapplications and patents disclose a novel method for producingself-supporting ceramic composites by growing an oxidation reactionproduct from a parent metal into a permeable mass of filler, therebyinfiltrating the filler with a ceramic matrix.

Further developments of the foregoing methods enable the formation ofceramic composite structures which (1) contain therein one or morecavities which inversely replicate the geometry of a shaped precursorparent metal, and (2) have a negative pattern which inversely replicatesthe positive pattern of a parent metal precursor. These methods aredescribed, respectively, (1) in Commonly Owned U.S. patent applicationSer. No. 823,542, which issued on May 9, 1989, as U.S. Pat. No.4,828,785 filed Jan. 27, 1986, in the name of Marc S. Newkirk et al.,entitled "Inverse Shape Replication Method of Making Ceramic CompositeArticles and Articles Obtained Thereby", and (2) in Commonly Owned U.S.patent application Ser. No. 896,157, filed Aug. 13, 1986, which issuedon Aug. 22, 1989, as U.S. Pat. No. 4,859,640, in the name of Marc S.Newkirk and entitled "Method of Making Ceramic Composite Articles withShape Replicated Surfaces and Articles Obtained Thereby."

A feature useful in the methods of the above-mentioned Commonly OwnedPatent Applications and patents to produce a net shape ceramic body,including composite bodies which retain essentially the original shapeand dimensions of the filler or preform, is to minimize or inhibitceramic matrix overgrowth of defined surface boundaries. Overgrowth ofthe surface boundaries can be substantially prevented by controlling theinfiltration of the polycrystalline ceramic matrix to any definedsurface boundaries, which may be accomplished such as by using apredetermined quantity of parent metal, establishing within the preformmore favorable oxidation kinetics than those outside the preform,exhausting the oxidizing atmosphere at some point in the process, orlowering the reaction temperature at some point in the process. Any ofthese steps may require close control or vigilance to obtain essentiallyno polycrystalline overgrowth of any defined surface boundary, and stillmay not produce the most desirable net or near net shape, or may requireadditional machining or finsihing to create acceptable tolerances in afinished part.

Methods were developed of making ceramic composite structures having apre-selected shape or geometry. These methods include the utilization ofa shaped preform of permeable filler into which the ceramic matrix isgrown by oxidation of a parent metal precursor, as described in CommonlyOwned U.S. patent application Ser. No. 338,471, filed on Apr. 14, 1989,as a continuation of U.S. patent application Ser. No. 861,025, filed May8, 1986, both in the names of Marc S. Newkirk et al. and entitled"Shaped Ceramic Composites and Methods of Making the Same". Anothermethod of making such shaped ceramic composites includes the utilizationof barrier means to arrest or inhibit the growth of the oxidationreaction product at a selected boundary to define the shape or geometryof the ceramic composite structure. This technique is described inCommonly Owned U.S. Pat. No. 4,923,832, which issued on May 8, 1990,from U.S. patent application Ser. No. 861,024, filed May 8, 1986, in thenames of Marc S. Newkirk et al. and entitled "Method of Making ShapedCeramic Composites with the use of a Barrier".

The entire disclosures of all of the foregoing Commonly Owned U.S.Patents and Patent Applications are expressly incorporated herein byreference.

The present invention provides another method for establishing a surfaceboundary on a ceramic composite which is desirable in forming net shapeceramic composites, particularly with larger, single-piece bodies orbodies with complicated geometry.

SUMMARY OF THE INVENTION

The present invention broadly provides a method for producing aself-supporting ceramic composite comprising a mass of filler material,such as a shaped preform, infiltrated by a ceramic matrix obtained bythe oxidation reaction of a parent metal to form a polycrystallinematrix material consisting essentially of the oxidation reaction productof the parent metal with one or more oxidants, including a vapor-phaseoxidant, and, optionally, one or more metallic constituents. Theself-supporting ceramic composite has a surface boundary, perimeter, orthe like, established by first providing on at least one surface of themass of filler material a permeable stratum or coating. The oxidationreaction process is continued to permit development or growth of theoxidation reaction product beyond the surface and into the stratum. Thisstratum with overgrowth of the matrix material beyond the mass of filleris predetermined or predesigned to be structurally weaker than theunderlying composite of infiltrated mass of filler, and can be easilymechanically removed or separated. Upon removal of this stratumcontaining this overgrowth from at least a portion of the surface, thereremains the exposed surface of the resulting composite in apredetermined shape.

More particularly with respect to the method of the present invention, aself-supporting ceramic composite is produced by contacting a zoneportion or extended surface of a mass of filler material with a body ofmolten metal obtained by heating a parent metal to a temperature aboveits melting point but below the melting point of the oxidation reactionproduct. The mass of filler may have a predetermined form or shape,either as a shaped preform bearing or surrounded by the permeablestratum as in the form of a loose bedding or coating, or by configuringthe stratum with a shaped surface which is then brought into engagementwith a mass of loose, conformable filler. At the aforedescribedtemperature or within this temperature range, the molten metal reactswith a vapor-phase oxidant to form the oxidation reaction product. Thevapor-phase oxidant may be used in conjunction with a solid oxidant or aliquid oxidant, as explained below in greater detail. The mass of fillermaterial has at least one surface with a stratum or coating of amaterial in conforming engagement with the surface, and the stratum isat least partially spaced from the contacting zone such that formationof the oxidation reaction product will occur into the mass of fillermaterial and in a direction towards and at least partially into thestratum. At least a portion of the oxidation reaction product ismaintained in contact with and between the molten metal and the oxidant,to draw molten metal through the oxidation reaction product towards theoxidant such that the oxidation reaction product continues to form atthe interface between the oxidant and previously formed oxidationreaction product that has infiltrated the mass of filler materialthereby forming a composite. The reaction is continued to permit growthbeyond the surface and into the stratum until at least a portion of thestratum has been infiltrated with the oxidation reaction product,thereby producing an intermediate ceramic body comprising the ceramicstratum and underlying ceramic composite, with the predeterminedinterface between the two defining the boundary or surface for theend-product. The stratum containing this overgrowth is predesigned to bestructurally or mechanically weaker than the underlying composite. Therelative mechanical integrities between the two layers is predeterminedas by a choice of materials and/or composition of the filler and thestratum, the array of these materials, the oxidation reaction productand its affinity for these materials, and one or more processconditions. This intermediate ceramic body, comprising the infiltratedstratum and adjacent composite, typically is cooled, and the ceramicstratum is removed or separated from the underlying composite by anysuitable mechanical means to produce a self-supporting ceramic compositehaving the defined surface established by the interface between thestratum and the infiltrated mass of filler.

The composite articles of this invention can be grown with substantiallyuniform properties throughout their cross-section to a thicknessheretofore difficult to achieve by conventional processes for producingdense ceramic structures. The process which yields these products alsoobviates the high costs associated with conventional ceramic productionmethods, including fine, high purity, uniform powder preparation, anddensification by sintering, hot pressing and/or hot isostatic pressing.

The products of the present invention are adaptable or fabricated foruse as articles of commerce which, as used herein, is intended toinclude, without limitation, industrial, structural and technicalceramic bodies for such applications where electrical, wear, thermal,structural, or other features or properties are important or beneficial;and is not intended to include recycle or waste materials such as mightbe produced as unwanted by-products in the processing of molten metals.

As used in this specification and the appended claims, the terms beloware defined as follows:

"Ceramic" is not to be unduly construed as being limited to a ceramicbody in the classical sense, that is, in the sense that it consistsentirely of non-metallic and inorganic materials, but rather refers to abody which is predominantly ceramic with respect to either compositionor dominant properties, although the body may contain minor orsubstantial amounts of one or more metallic constituents derived fromthe parent metal or produced from the oxidant or by dopant, mosttypically within a range of from about 1-40% by volume, but may includestill more metal.

"Oxidation reaction product" generally means one or more metals in anyoxidized state wherein a metal has given up electrons to or sharedelectrons with another element, compound, or combination thereof.Accordingly, an "oxidation reaction product" under this definitionincludes the product of reaction of one or more metals with an oxidant.

"Oxidant" means one or more suitable electron acceptors or electronsharers and may be an element, a combination of elements, a compund, ora combination of compounds, including reducible compounds, and is vapor,solid, or liquid at the process conditions.

"Parent metal" refers to that metal, e.g., aluminum, which is theprecursor for the polycrystalline oxidation reaction product, andincludes that metal as a relatively pure metal, a commercially availablemetal with impurities and/or alloying constituents, or an alloy in whichthat metal precursor is the major constituent; and when a specifiedmetal is mentioned as the parent metal, e.g., aluminum, the metalidentified should be read with this definition in mind unless indicatedotherwise by the context.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic, vertical cross-sectional view showing an assemblyof a parent metal ingot in a suitable bedding overlaid by a preformbearing a permeable stratum, and confined within a refractory vessel.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

In accordance with a preferred embodiment of the present invention, theparent metal, which may be doped (as explained below in greater detail)and is the precursor to the oxidation reaction product, is formed intoan ingot, billet, rod, plate, or the like, and placed in an inert bed,crucible, or other refractory container. A permeable shaped preform(described below in greater detail) is formed or manufactured such as tohave at least one defined surface boundary and to be permeable to thevapor-phase oxidant and to the infiltration oxidation reaction product.The preform is placed adjacent to and preferably in contact with one ormore surfaces of, or a portion of a surface of, the parent metal suchthat at least a portion of the defined surface boundary of the preformis generally positioned distantly or outwardly or spaced from the metalsurface of the parent metal. The preform preferably is in contact withan areal surface of the parent metal; but when desired, the preform maybe partially immersed, but not totally immersed, in the molten metalbecause complete immersion would cut off or block development of thepolycrystalline matrix.

A permeable stratum is formed, applied or spread as a coating or layeronto the preform to have at least one surface that is substantiallyconformable to the geometry of the defined surface boundary of thepreform. The stratum is sufficiently porous to be permeable to thevapor-phase oxidant and to the infiltrating oxidation reaction product.The permeable stratum, which need not be uniform in thickness, has itsconformed surface contiguous with, or bearing against, the definedsurface boundary of the preform. Formation of the oxidation reactionproduct will occur in a direction towards the defined surface boundaryand the permeable stratum which establishes the surface, perimeter orboundary of the ceramic composite. The container and its contents aresubsequently placed in a furnace which is supplied with an oxidant,including a vapor-phase oxidant. This setup is heated to temperaturesbelow the melting point of the oxidation reaction product but above themelting point of the parent metal, which for, example, in the case ofaluminum using air as the vapor-phase oxidant, is generally betweenabout 850°-1450° C. and more preferably between about 900°-1350° C.Within this temperature interval or preferred temperature range, a bodyor pool of molten metal forms, and on contact with the oxidant(s), themolten metal will react to form a layer of oxidation reaction product.Upon continued exposure to the oxidizing environment, molten metal isprogressively drawn into and through any previously formed oxidationreaction product in the direction of the oxidant and towards the definedsurface boundary that is in contact with the permeable stratum. Oncontact with the oxidant, the molten metal will react to form additionaloxidation reaction product while, optionally, leaving metallicconstituents dispersed through the polycrystalline material. At least aportion of the oxidation reaction product is maintained in contact withand between the molten parent metal and the oxidant(s) to sustain thecontinued growth of the polycrystalline oxidation reaction product inthe preform. The polycrystalline oxidation reaction product willcontinue to grow and develop within the preform, embedding itsconstituents. The process is continued until the oxidation reactionproduct has grown beyond the defined surface boundary into at least aportion of the permeable stratum to produce an intermediate ceramic bodycomprising an underlying ceramic composite body that has beeninfiltrated with the oxidation reaction product and a ceramic stratumthat at least partially has been infiltrated with the oxidation reactionproduct. In conducting the process, it is predetermined for theresulting ceramic stratum to have a mechanical integrity integrity thatis weaker or less substantial mechanically than the mechanical integrityof the ceramic composite body. "Mechanical integrity" may be defined asthat quality or strength in the respective ceramic structures thatallows the ceramic stratum to be removed, such as by grit-blasting,tubling in abrasive media, or slurry erosion technique, withoutdisturbing or disrupting the underlying ceramic composite which stayssubstantially intact while the ceramic stratum is being removed andafter it is removed.

The intermediate ceramic body, comprising the stratum and filler bothinfiltrated with oxidation reaction product, is removed from thefurnace, and is allowed to cool below about 850° C., preferably belowabout 400° C. to about room temperature. In a preferred embodiment, oncooling, the composite ceramic stratum will develop microcracks in itsceramic matrix due to martensitic phase transformation of stratumconstituents entrained within the grown matrix, resulting in the ceramicstratum being easier to remove from the ceramic composite body than ifthe intermediate ceramic body was not cooled. The microcracked compositeceramic stratum is subsequently removed, such as by an erosiontechnique, from the ceramic composite body.

The permeable stratum may comprise any material(s), compound(s), or thelike, compatible with the growth of the oxidation reaction productmatrix therein and has a mechanical integrity after being infiltratedwith the oxidation reaction product, that is weaker or less substantialmechanically than the mechanical integrity of the underlying compositebody in order that the permeable stratum, including any infiltratedoxidation reaction product, may be easily and preferentially eroded awayfrom or otherwise removed from the underlying composite body withoutaffecting the latter, such as by cracking, pitting, or the like. Thepermeable stratum also may comprise any material(s), compound(s), or thelike, that on post-process cooling develops microcracks due tomartensitic phase transformation resulting from the stratum beingunstabilized or becoming unstabilized during the oxidation reactiongrowth process. The composition of the stratum will depend largely onthe composition of the preform and the developed ceramic matrix, butalso can depend on the oxidant and the process conditions. The materialsand reaction conditions are pre-selected so that the infiltraed stratumcomposite is weaker than the adjacent infiltrated filler composite andthat the stratum can be easily separated at the interface. In apreferred embodiment of the invention, utilizing aluminum as the parentmetal and air as the oxidant to form an alpha-alumina matrix, thepermeable stratum comprises an unstabilized compound selected from thegroup consisting of zirconia, hafnia, and mixtures thereof. Moreparticularly, if the permeable stratum comprises unstabilized zirconiaand the filler alumina, the stratum infiltrated with the aluminaoxidation reaction product is mechanically weaker than the adjacentinfiltrated bed, and can be readily separated from the bed at theinterface by grit-blasting, polishing, slurry erosion, or the like.

The permeable stratum that is positioned contiguously with respect tothe defined surface boundary of the preform may be any suitable form ormaterial, such as a coating, bedding, or the like, of platelets, wires,particulates, powders, bubbles, etc., and combinations thereof. Thematerial may be bonded with any suitable binding agent to provide greenstrength, e.g. polyvinyl alcohol or the like, that does not interferewith the reactions of this invention. Larger particulates having a meshsize of, for example, 24 mesh or larger are particularly useful becauseof their tendency to form very weak composites. Finer sizes, however,may be employed, including admixtures of mesh sizes. The particulatematerial or compound of the permeable stratum may be conformed or moldedto the preform surface by known or conventional techniques as by forminga slurry of the particulate in an organic binder, applying the slurry tothe surface, and then letting the part set as by drying at elevatedtemperatures.

The resulting self-supporting ceramic composite as the final product isinfiltrated or embedded to its boundaries by a ceramic matrix comprisinga polycrystalline material consisting essentially of the oxidationreaction product of the parent metal with the vapor-phase oxidant and,optionally, one or more metallic constituents such as non-oxidizedconstituents of the parent metal, dopants, or metallic constituents of areducible oxidant. Most typically, the boundaries of the bed of filleror filler preform and of the polycrystalline matrix substantiallycoincide; but individual constituents at the surfaces of the bed orpreform may be exposed or may protrude from the matrix, and thereforeinfiltration and embedment may not completely surround or encapsulatethe filler by the matrix. It further should be understood that theresulting polycrystalline matrix may exhibit porosity which may be apartial or nearly complete replacement of the metal phase, but thevolume percent of voids will depend largely on such conditions astemperature, time, type of parent metal, and dopant concentrations.Typically in these polycrystalline ceramic structures, the oxidationreaction product crystallites are interconnected in more than onedimension, preferably in three dimensions, and the metal phase or porephase may be at least partially interconnected. The ceramic compositeproduct of this invention has generally well-defined boundaries. Thus,the permeable stratum establishes a boundary of the self-supportingceramic composite and assists in producing a well-defined, net or nearnet shaped self-supporting ceramic composite.

The ceramic composite obtained by the practice of the present inventionwill usually be a coherent product wherein between about 5% and about98% by volume of the total volume of the ceramic composite product iscomprised of one or more of the filler materials embedded to the definedsurface boundary of the preform or bed with a polycrystalline matrix.The polycrystalline matrix is usually comprised of, when the parentmetal is aluminum, about 60% to about 99% by volume (of the volume ofpolycrystalline matrix) of interconnected alpha-alumina oxide and about1% to 40% by volume (same basis) of nonoxidized constituents of theparent metal.

Although the present invention is hereinafter described with particularemphasis on systems wherein aluminum or an aluminum alloy is employed asthe parent metal and alumina is the intended oxidation reaction product,this reference is for exemplary purposes only, and it is to beunderstood that the present invention is adaptable by application of theteachings herein to other systems wherein other metals such as tin,silicon, titanium, zirconium, etc., are employed as the parent metal,and the intended oxidation reaction product is that metal oxide,nitride, boride, carbide, or the like. Also, the invention is describedbelow with particular reference to a preform in the formation ofcomposite bodies, but it should be understood that any loose fillerbeds, materials, or the like, with at least one defined surface boundaryare also applicable and useful in the practice of this invention. Thus,whenever "preform" or "permeable preform" is referred to herein, it isto be construed to mean any mass of filler or filler material that ispermeable to the vapor-phase oxidant and the oxidation reaction growthprocess of this invention and has at least one defined surface.

Referring now to the drawing for further describing the invention by wayof example only, a parent metal 10 is embedded in a substantially inertfiller 12 such that the top surface of the metal is substantially flushwith the bedding. A preform 14 having a predetermined shaped surfaceindicated generally at 16 is placed on the top surface of the parentmetal. A permeable stratum 18 is applied to surface 16 withoutdisturbing or upsetting the geometry of this surface. This lay-up iscontained in a suitable refractory vessel or boat 20. It will beobserved that the assembly is arranged so that the growth or developmentof the oxidation reaction product will occur into the preform 14 and ina direction towards the defined surface boundary 16. The oxidationreaction product infiltrates or engulfs the preform 14 and at least aportion of the permeable stratum 18. The assembly is heated in a furnace(not shown) to an elevated temperature in the presence of a vapor-phaseoxidant as previously described so that the polycrystalline ceramicgrowth infiltrates the preform beyond the defined surface boundary 16and into at least a portion of the permeable stratum 18 withoutsubstantially disturbing or displacing the preform 14, in order toproduce an intermediate ceramic body. The intermediate ceramic bodycomprises a ceramic stratum (the stratum infiltrated by thepolycrystalline ceramic growth) overlaying a ceramic composite body (thepreform infiltrated by the polycrystalline ceramic growth). The ceramicstratum exhibits a mechanical integrity that is weaker or lesssubstantial mechanically than the mechanical integrity of the ceramiccomposite body, and the ceramic stratum may be removed such as bygrit-blasting, etc., from the ceramic composite body without affectingthe mechanical integrity or structure of the latter. Typically, theintermediate ceramic body is allowed to cool as by removing the lay-upfrom the furnace before separating the ceramic stratum from theunderlying ceramic composite body. Upon removal of the ceramic stratumalong the defined surface boundary 16, the resulting ceramic product isa self-supporting ceramic composite having the defined surfaceestablished by the permeable stratum 18.

In the process of this invention, the vapor-phase oxidant is normallygaseous or vaporized at the process conditions to provide an oxidizingatmosphere, such as atmospheric air. Typical vapor-phase oxidantsinclude, for example, elements or compounds of the following, orcombinations of elements or compounds of the following, includingvolatile or vaporizable elements, compounds or constituents of compoundsor mixtures: oxygen, nitrogen, a halogen, sulfur, phosphorus, arsenic,carbon, boron, selenium, tellurium, and compounds and combinationsthereof, for example, methane, ethane, propane, acetylene, ethylene,propylene (the hydrocarbon as a source of carbon), and mixtures such asair, H₂ /H₂ O and a CO/CO₂, the latter two (i.e., H₂ /H₂ O and CO/CO₂)being useful in reducing the oxygen (including air) with air usuallybeing more preferred for obvious reasons of economy. When a vapor-phaseoxidant is identified as containing or comprising a particular gas orvapor, this means a vapor-phase oxidant in which the identified gas orvapor is the sole, predominant or at least a significant oxidizer of theparent metal under the conditions obtained in the oxidizing environmentutilized. For example, although the major constituent of air isnitrogen, the oxygen content of air is normally the sole oxidizer of theparent metal under the conditions obtained in the oxidizing environmentutilized. Air therefore falls within the definition of an"oxygen-containing gas" oxidant but not within the definition of a"nitrogen-containing gas" oxidant. An example of a "nitrogen-containinggas" oxidant as used herein and in the claims is "forming gas", whichtypically contains about 96 volume percent nitrogen and about 4 volumepercent hydrogen.

The oxidant may also include a solid oxidant and/or a liquid oxidant,which is solid or liquid at the process conditions. The solid oxidantand/or the liquid oxidant is employed in combination with thevapor-phase oxidant. When a solid oxidant is employed, it is usuallydispersed or admixed through the entire filler bed or preform or througha portion of the bed or preform adjacent the parent metal, inparticulate form, or perhaps as a coating on the bed or preformparticles. Any suitable solid oxidant may be employed includingelements, such as boron or carbon, or reducible compounds, such asoxides, carbides, or borides of lower thermodynamic stability than theoxide or boride reaction product of the parent metal.

If a liquid oxidant is employed in conjunction with the vapor-phaseoxidant, it may be dispersed throughout the entire filler bed or preformor a portion thereof adjacent to the parent metal, provided such liquidoxidant does not block access of the molten metal to the vapor-phaseoxidant. Reference to a liquid oxidant means one which is a liquid underthe oxidation reaction conditions and so a liquid oxidant may have asolid precursor such as a salt, which is molten or liquid at theoxidation reaction conditions. Alternatively, the liquid oxidant may bea liquid precursor, e.g., a solution of a material, which is used tocoat part or all of the porous surfaces of the filler bed or preform andwhich is melted or decomposed at the process conditions to provide asuitable oxidant moiety. Examples of liquid oxidants as herein definedinclude low melting glasses.

The preform should be sufficiently porous or permeable to allow thevapor-phase oxidant to permeate the preform and contact the parentmetal. The preform also should be sufficiently permeable to accommodategrowth of the oxidation reaction product within the preform withoutsubstantially disturbing, upsetting, or otherwise altering theconfiguration or geometry of the preform. In the event the preformincludes a solid oxidant and/or liquid oxidant which may accompany thevapor-phase oxidant, the preform then should be sufficiently porous orpermeable to permit and accept growth of the oxidation reaction productoriginating from the solid and/or liquid oxidant. It should beunderstood that whenever "preform" or "permeable preform" is referred toherein, it means a permeable preform possessing the foregoing porosityand/or permeability properties unless otherwise stated.

The permeable preforms may be created or formed into any predetermineddesired size and shape by any conventional methods, such as slipcasting,injection molding, transfer molding, vacuum forming, or otherwise, byprocessing any suitable material(s), more specifically identified anddescribed elsewhere. The permeable preform, as was previously mentioned,may include a solid oxidant and/or liquid oxidant, used in conjunctionwith a vapor-phase oxidant as the oxidant. The permeable preform shouldbe manufactured with at least one surface boundary, and such as toretain a significant shape integrity and green strength, as well asdimensional fidelity after being infiltrated and embedded by the ceramicmatrix. The permeable preform, however, should be permeable enough toaccept the growing polycrystalline oxidation reaction product. Thepermeable preform should also be capable of being wetted by the parentmetal, and of such constituency that the polycrystalline oxidationreaction product can bond or adhere to and within the preform to producea ceramic composite product of high integrity and well-defined borders.

The preform may be of any size or shape, as long as it contacts or isadjacent to or in extended surface contact with the metal surface of theparent metal and has at least one surface boundary with a superimposedpermeable stratum which defines a destination for the growingpolycrystalline matrix. By way of example only, the preform may behemispherical in shape with the flat surface boundary in contact withthe parent metal surface and the dome-shaped surface boundaryrepresenting the defined surface boundary to where the polycrystallinematerial is to grow; or the preform may be cubical in shape with onesquare surface boundary contacting the metal surface of the parent metaland the remaining five square surface boundaries being the objectivepoints for the growing polycrystalline matrix. A matrix of thepolycrystalline material resulting from the oxidation reaction productis simply grown into the permeable preform and into the stratum so as toinfiltrate and embed the preform to its defined surface boundary and atleast partially infiltrate the contiguously disposed permeable stratum,without substantially disturbing or displacing the permeable preform.

The permeable preform of this invention may be composed of any suitablematerial, such as ceramic and/or metal particulates, powders, fibers,whiskers, wires, particles, hollow bodies or spheres, wire cloth, solidspheres, etc., and combinations thereof. The preform materials cancomprise either a loose or bonded array or arrangement, which array hasinterstices, openings, intervening spaces, or the like, to render thepreform permeable to the oxidant and the infiltration of molten parentmetal to allow for the formation of oxidation reaction product growthwithout altering the configuration of the preform. The preform mayinclude a lattice of reinforcing rods, bars, tubes, tubules, plates,wire, spheres or other particulates, wire cloth, ceramic refractorycloth or the like, or a combination of any of the foregoing, prearrangedin a desired shape. Further, the material(s) of the preform may behomogeneous or heterogeneous. The suitable materials of the preform may,such as ceramic powders or particulate, be bonded together with anysuitable binding agent, or the like, which does not interfere with thereactions of this invention, or leave any undesirable residualby-products within the ceramic composite product. Suitable particulates,such as silicon carbide or alumina, may have a grit size of from about10 to 1000 or smaller or an admixture of grit sizes and types may beused. The particulate may be molded by known or conventional techniquesas by forming a slurry of the particulate in an organic binder, pouringthe slurry into a mold, and then letting the mold set as by drying orcuring at an elevated temperature.

Any of a number of suitable materials may be employed in the formationand manufacture of the preform or filler bed. Such suitable materialsinclude those which, under the temperature and oxidizing conditions ofthe process, are not volatile, are thermodynamically stable and do notreact with or dissolve excessively in the molten parent metal. Someuseful filler materials can be provided with a protective coating torender the material stable and to avoid unwanted reactions. Wherealuminum is the parent metal and air or oxygen is employed as theoxidant, such materials include, for example, the metal oxides, borides,nitrides, and carbides of aluminum, cerium, hafnium, lanthanum,praseodymium, samarium, zirconium, and higher order metallic compoundssuch as magnesium aluminate spinel, and coated carbon fibers. Certain ofthese constituents may have to be coated with an oxidation protectivecoating in order to survive the oxidizing conditions of the process. Insuch case, the coating must be compatible with the development of thematrix.

A preform used in the practice of this invention may be employed as asingle preform or as an assemblage of preforms to form more complexshapes. It has been discovered that the polycrystalline matrix materialcan be grown through adjacent, contacting portions of a preformassemblage to bond contiguous preforms into a unified, or integral,ceramic composite. The assembly of preforms, provided with a permeablestratum at the surface(s), is arranged so that a direction of growth ofthe oxidation reaction product will be towards and into the assembly ofpreforms to infiltrate and embed the assembly, and the permeablestratum, thereby bonding the preforms together. Thus, complex and shapedceramic composites can be formed as an integral body which cannototherwise be produced by conventional manufacturing techniques. Itshould be understood that whenever "preform" is referred to herein, itmeans a preform or an assemblage of preforms (unless otherwise stated)which may be ultimately bonded into an integral composite.

As a further embodiment of the invention and as explained in theCommonly Owned Patent Applications and Patents, the addition of dopantmaterials in conjunction with the parent metal can favorably influenceor promote the oxidation reaction process. The function or functions ofthe dopants can depend upon a number of factors other than the dopantmaterial itself. These factors include, for example, the particularparent metal, the end product desired, the particular combination ofdopants when two or more dopants are used, the use of an externallyapplied dopant in combination with an alloyed dopant, the concentrationof the dopant, the oxidizing environment, and the process conditions.

The dopant or dopants used in conjunction with the parent metal (1) maybe provided as alloying constituents of the parent metal, (2) may beapplied to at least a portion of the parent metal, or (3) may be appliedto the filler bed or preform or to a part thereof, e.g., the supportzone of the preform, or any combination of two or more techniques (1),(2), and (3) may be employed. For example, an alloyed dopant may be usedin combination with an externally applied dopant. In the case oftechnique (3) where a dopant or dopants are applied to the filler bed orpreform, the application may be accomplished in any suitable manner,such as by dispersing the dopants throughout part of the entire mass ofthe preform as coatings or in particulate form, preferably including atleast a portion of the preform adjacent the parent metal. Application ofany of the dopants to the filler may also be accomplished by applying alayer of one or more dopant materials to and within the preform,including any of its internal openings, interstices, passageways,intervening spaces, or the like, that render it permeable. A convenientmanner of applying any of the dopant material is to merely soak thefiller to be employed in a liquid source (e.g., a solution of dopantmaterial).

A source of the dopant may also be provided by placing a rigid body ofdopant in contact with and between at least a portion of the parentmetal surface and the preform. For example, a thin sheet ofsilica-containing glass (useful as a dopant for the oxidation of analuminum parent metal) can be placed upon a surface of the parent metal.When the aluminum parent metal (which may be internally doped with Mg)overlaid with the silicon-containing material is melted in an oxidizingenvironment (e.g., in the case of aluminum in air, between about 850° C.to about 1450° C., preferably about 900° C. to about 1350° C.), growthof the polycrystalline ceramic matrix material into the permeablepreform occurs. In the case where the dopant is externally applied to atleast a portion of the surface of the parent metal, the polycrystallineoxide structure generally grows within the permeable preformsubstantially beyond the dopant layer (i.e., to beyond the depth of theapplied dopant layer). In any case, one or more of the dopants may beexternally applied to the parent metal surface and/or to the permeablepreform. Additionally, dopants alloyed within the parent metal and/orexternally applied to the parent metal may be augmented by dopant(s)applied to the aforementioned forms. Thus, any concentrationdeficiencies of the dopants alloyed within the parent metal and/orexternally applied to the parent metal may be augmented by additionalconcentration of the respective dopant(s) applied to the preform andvice versa.

Useful dopants for an aluminum parent metal, particularly with air asthe oxidant, include, for example, magnesium metal and zinc metal, incombination with each other or in combination with other dopants asdescribed below. These metals, or a suitable source of the metals, maybe alloyed into the aluminum-based parent metal at concentrations foreach of between about 0.1-10% by weight based on the total weight of theresulting doped metal. Concentrations within this range appear toinitiate the ceramic growth, enhance metal transport and favorablyinfluence the growth morphology of the resulting oxidation reactionproduct. The concentration for any one dopant will depend on suchfactors as the combination of dopants and the process temperature.

One or more dopants may be used depending upon the circumstances, asexplained above. For example, in the case of an aluminum parent metaland with air as the oxidant, particularly useful combinations of dopantsinclude (a) magnesium and silicon or (b) magnesium, zinc, and silicon.In such examples, a preferred magnesium concentration falls within therange of from about 0.1 to about 3% by weight, for zinc in the range offrom about 1 to about 6% by weight, and for silicon in the range of fromabout 1 to about 10% by weight.

Additional examples of dopant materials, useful with aluminum parentmetal, include sodium, lithium, calcium, boron, phosphorus and yttrium,which may be used individually or in combination with one or more otherdopants depending on the oxidant and process conditions. Sodium andlithium may be be used in very small amounts in the parts per millionrange, typically about 100-200 parts per million, and each may be usedalone or together, or in combination with other dopant(s). Rare earthelements such as cerium, lanthanum, praseodymium, neodymium and samariumare also useful dopants, and herein again especially when used incombination with other dopants.

As noted above, it is not necessary to alloy any dopant material intothe parent metal. For example, selectively applying one or more dopantmaterials in a thin layer to either all or a portion of the surface ofthe parent metal enables local ceramic growth from the parent metal orportions thereof and lends itself to growth of the polycrystallineceramic material into the permeable preform in selected areas. Thus,growth of the polycrystalline ceramic matrix material into the permeablepreform can be controlled by the localized placement of the dopantmaterial upon the surface of the parent metal. The applied coating orlayer of dopant is thin relative to the thickness of the parent metalbody, and growth or formation of the oxidation reaction product into thepermeable preform extends to substantially beyond the dopant layer. Suchlayer of dopant material may be applied by painting, dipping, silkscreening, evaporating, or otherwise applying the dopant material inliquid or paste form, or by sputtering, or by simply depositing a layerof a solid particulate dopant or a solid thin sheet or film of dopantonto the surface of the parent metal. The dopant material may, but neednot, include either organic or inorganic binders, vehicles, solvents,and/or thickeners. More preferably, the dopant materials are applied aspowders to the surface of the parent metal or dispersed through at leasta portion of the filler. One particularly preferred method of applyingthe dopants to the parent metal surface is to utilize a liquidsuspension of the dopants in a water/organic binder mixture sprayed ontoa parent metal surface in order to obtain an adherent coating whichfacilitates handling of the doped parent metal prior to processing.

The dopant materials when used externally are usually applied to aportion of the surface of the parent metal as to uniform coatingthereon. The quantity of dopant is effective over a wide range relativeto the amount of parent metal to which it is applied, and in the case ofaluminum, experiments have failed to identify either upper or loweroperable limits. For example, when utilizing silicon in the form ofsilicon dioxide externally applied as a dopant for an aluminum-magnesiumparent metal using air or oxygen as the oxidant, quantities as low as0.00003 gram of silicon per gram of parent metal, or about 0.0001 gramof silicon per square centimeter of exposed parent metal surface,together with a second dopant having a source of magnesium and/or zinc,to produce the polycrystalline ceramic growth phenomenon. It also hasbeen found that a ceramic structure is achievable from an aluminum-basedparent metal using air or oxygen as the oxidant by using MgO as a dopantin an amount greater than about 0.0008 gram of Mg per gram of parentmetal to be oxidized and greater than about 0.003 gram of Mg per squarecentimeter of parent metal surface upon which the MgO is applied. Itappears that to some degree an increase in the quantity of dopantmaterials will decrease the reaction time necessary to produce theceramic composite, but this will depend upon such factors as type ofdopant, the parent metal, and the reaction conditions.

Where the parent metal is aluminum internally doped with magnesium andthe oxidizing medium is air or oxygen, it has been observed thatmagnesium is at least partially oxidized out of the alloy attemperatures of from about 820° C. to 950° C. In such instances ofmagnesium-doped systems, the magnesium forms a magnesium oxide and/ormagnesium aluminate spinel phase at the surface of the molten aluminumalloy, and during the growth process such magnesium compounds remainprimarily at the initial oxide surface of the parent metal alloy (i.e.,the "initial surface") in the growing ceramic structure. Thus, in suchmagnesium-doped systems, an aluminum oxide-based structure is producedapart from the relatively thin layer of magnesium aluminate spinel atthe initiation surface. Where desired, this initiation surface can bereadily removed as by grinding, machining, polishing, or grit-blasting.

The invention is further illustrated by the following example.

EXAMPLE

A one inch thick by seven-eighth inch wide by eight inch long ingot ofaluminum alloy comprising 5% silicon, 3% magnesium, 91.7% aluminum,balance impurities, all by weight, is placed horizontally upon a layerof relatively inert material of 38 Alundum, 100 mesh size (by NortonCompany), contained within a crucible. The ingot is subsequently coveredwith a preform having a defined surface boundary. The preform may befabricated by conventional slip casting technique, and is made from aslurry comprising 47.6% alumina particles (E67 Alundum, from Norton Co.,1000 mesh size), 23.7% Kaolin clay (EPK, Georgia Kaolin, 98% less than20 um particle size) and 28.5% water, is mixed uniformly, and pouredinto a plaster of paris mold having the desired geometry of the preform.The crucible preform is cast for approximately 20 minutes, dried at 90°C. and then prefired at 700° C. for 30 minutes in air. The preform iscovered with zirconia, for example 24 mesh, at its defined surfaceboundary to a depth of approximately three inches. The lay-up is placedin a furnace (vented to allow for the flow of air), which is at 1000° C.and is held there for 96 hours to produce a ceramic composite bodyoverlaid with zirconia stratum that is infiltrated with oxidationreaction product. The zirconia stratum exhibits a mechanical integrityweaker than a mechanical integrity of the ceramic composite body. Aftercooling, the zirconia stratum is removed by grit-blasting to produce aself-supporting ceramic composite having the defined surface boundaryestablished by the zirconia stratum.

What is claimed is:
 1. A method for producing a self-supporting ceramiccomposite body comprising a mass of filler material infiltrated by aceramic matrix, said ceramic matrix being obtained by the oxidation of aparent metal to form a polycrystalline matrix comprising an oxidationreaction product of the parent metal, said method comprising:(A) heatinga parent metal to a temperature above its melting point but below themelting point of its oxidation reaction product to form a body of moltenmetal; (B) contacting said body of molten metal with a permeable mass offiller material having at least one surface bearing a stratum, therebyforming an interface between the molten metal and the filler material,said stratum (a) substantially conforming to the geometry of at least aportion of said at least one surface of said mass of filler material,(b) being permeable to a vapor-phase oxidant, (c) comprising anunstabilizable material, and (d) being permeable to infiltration bygrowth of the oxidation reaction product, and said stratum being atleast partially spaced from said interface such that formation of saidoxidation reaction product will occur into said mass of filler materialand in a direction toward and at least partially into said stratum; andat said temperature:(i) reacting said molten metal with an oxidant toform an oxidation reaction product, (ii) maintaining at least a portionof said oxidation reaction product in contact with and between saidmolten metal and said oxidant, to progressively transport molten metalthrough the oxidation reaction product toward the oxidant so that freshoxidation reaction product continues to form at an interface betweensaid oxidant and previously formed oxidation reaction product that hasinfiltrated said mass of filler material to produce a ceramic compositebody, and (iii) continuing said reacting to infiltrate at least aportion of said stratum with said oxidation reaction product to producea ceramic stratum overlaying said ceramic composite body; and (C)recovering a self-supporting ceramic composite body having said at leastone surface established by said stratum.
 2. The method according toclaim 1, further comprising shaping said filler material into a preform.3. The method according to claim 1, wherein said filler material furthercomprises at least one material selected from the group consisting ofsolid and liquid oxidants.
 4. The method according to claim 1, furthercomprising introducing said stratum in a manner sufficient to result insaid ceramic stratum which possesses a mechanical integrity that is lessthan said ceramic composite.
 5. The method according to claim 1, whereinsaid stratum comprises a loose bedding.
 6. The method according to claim1, wherein said stratum comprises a coating.
 7. The method according toclaim 1, wherein said parent metal comprises at least one materialselected from the group consisting of aluminum, tin, silicon, titanium,zirconium, and hafnium.
 8. The method according to claim 1, wherein saidstratum comprises at least one material selected from the groupconsisting of zirconia and hafnia.